The present invention relates to a friction vacuum pump, comprising a housing, with a shaft which is supported in the housing by bearings and with a rotor which has a central borehole and which is mounted onto the shaft in the area of said borehole.
Friction vacuum pumps, turbomolecular pumps in particular, are operated at very high speeds (up to 100,000 revolutions per minute). Such operation requires extremely precise balancing of the rotor unit consisting of shaft and rotor. In spite of involved balancing procedures, even so, vibrations (vibration accelerations) were observed ever so often which occurred after a few or after many operating hours and the cause of which was initially unclear.
The inventors have set themselves the target of reducing these interfering vibration effects. Through the measures detailed in the patent claims they have attained this target.
The solutions stated are based on the realisation that the joint area between rotor and shaft is frequently a source of displacements and thus the vibration effects observed. In this area, loosening may occur during the operation of a friction vacuum pump of the kind affected here, for two reasons. One of the reasons is that the occurring centrifugal forces also have an effect on the joint area. A further cause for loosening in the joint area is based on the fact that the rotor material is commonly aluminium and that for the shaft is steel (with a smaller coefficient of expansion compared to aluminium). In the case of increases in temperature, the shaft is not capable of following the temperature induced movements of the rotor. In both instances slotsxe2x80x94be they even very minutexe2x80x94are created between shaft and rotor thus causing imbalances. Commonly the phenomena detailed superimpose. Through a tighter fit the effects detailed might be diminished. The selection of a sufficiently tight fits is, however, not possible since the fit between rotor and shaft must, in the radial direction, not be so tight rendering assembly impossible, resp. the required axial forces for bracing the unit are consumed by way of friction.
Through the measures proposed through the patent claims, the disadvantageous effects detailed are eliminated. Deformations affecting the rotor in the area of the shaftxe2x80x94be they caused by the effect of centrifugal forces or owing to temperature induced movementsxe2x80x94can be substantially avoided. As a result this will generally significantly reduce the vibration effects observed, frequently to an extent that these are entirely removed.
Through a ring groove at least in the area of one of the two front faces of the rotor, the effect is attained that the occurring centrifugal forces which attain high values in the area of the rotor""s periphery will be ineffectual at least in the area of that rotor section which forms the border of the ring groove according to the present invention towards the inside. At least deformations resulting in loosening within the joint area are substantially prevented in this area. If this area is in addition equipped with a reinforcing ring, then the risk of loosening in the joint area for thermal reasons is also eliminated. Preferably the material the reinforcing ring is made of has a relatively small thermal coefficient of expansion.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.